I have a 1216H switch to control my Expert PA. I use a ethernet power switch to turn on the RemoteRig box, transceiver power supply and 1216H WebSwitch. Rear PA switch is always ON.When I power on the webswitch the PA turns On (as If I press the ON front panel button) and I need to turn it off via the web interface. Is there any way to avoid this behaviour?. I guess it have something to do with the serial port, as I suffered the same behaviour when connecting the PA to a RaspberryPi.

It is probably not to be avoided. It depends upon which amp you have. the 1K-FA uses a serial signal from the rig to turn on, so turning that off should also turn off the amp. The 2K, 1.2K and 1.3Kneed 12v to the connector on the back to turn on, so if you only have one plug, it can't be avoided.